In the late 1960's and early 1970's, an improved water-impermeable membrane was developed to protect concrete bridge decks and roadways from attack by road salts and freeze-thaw cycling degradation. This improved membrane formed the subject matter claimed in U.S. Pat. No. 3,870,426 issued Mar. 11, 1975, which is incorporated herein by reference. That membrane could be applied by conventional road paving equipment without the need for special surface preparation, was capable of healing its own cracks and could tolerate temporary traffic until a wearing course could be laid thereover. However, that membrane contained asbestos fibers which are known, in certain quantities and fiber sizes, to pose significant health hazards, particularly to smokers. Because the addition of the asbestos fibers to the mix was generally performed on-site outside its control, the Assignee dedicated the remaining life of the patent, so as not to have to license such technology or sell fiber for such usage, and has sought a membrane formulation with equivalent or better characteristics without the associated health hazards.
The asbestos membrane required asphalt concentrations of 14-18% (40 to 50% above normal) or higher to assure sufficient compactability and crack resistance, asbestos fiber asphalt ratios above 0.30 preferably from 0.35 to 0.45 for stability, and overall fiber quantities between 5 and 7%. For that membrane mix, stability, as measured by plastic flow, was virtually independent of asphalt content, being primarily a function of cohesiveness of the mix which was largely dependent on fiber content (hence, the minimum fiber/asphalt ratio).
Initial success at stabilizing high asphalt-containing pavements as in Ser. No. 343,075 ('075) above, suggested that diatomite might prove suitable as a filler in a membrane mix. Field tests of small pavement sheets have shown diatomite to be even more effective in such membrane mixes than was asbestos. Because of its high surface area, microscopic porous structure, and affinity for absorbing asphalt (i.e., effectiveness at resisting flow), diatomite can be used in small quantities (0.5 to 3.0%) in hot mix membranes to achieve stability. In such small amounts, diatomite does not cause the high viscosity which adversely affects compaction and permeability of the membrane mix typical of 5-7% asbestos (or, a like amount of diatomite, for that matter). In fact, small amounts of diatomite function as an extender improving compactability as described and claimed in application Ser. No. 06/457,984 ('984). Accordingly, it is unnecessary to increase asphalt content 50% to the 14-18% by weight range in order to improve compaction and achieve impermeability of the membrane. A much more modest increase of 5 to 35% percent above normal sheet asphalt concentration of 8.5 to 10.5% to the 9 to 14% by weight range is made possible in the diatomite-modified mix.
Field trials indicate that the diatomite-modified membrane mix will have all of the advantages of the asbestos membrane and several others, in addition. This membrane is less expensive than the asbestos membrane, since it uses less asphalt and less filler, and is significantly easier to place. Unlike the asbestos mix, the stability of the diatomite-modified membrane mix is not independent of asphalt content. In addition, diatomite's influence on plastic flow is not limited to increasing cohesiveness but also relates to its effects on interparticle friction. Further, for hot mixes using conventional asphalts (80-110 pen.) and CELITE.RTM.292, there does not appear to be any appreciable benefit to stability obtained by increasing the diatomite/asphalt ratio above 0.16. (Note, placability and/or the use of other diatomites may dictate higher diatomite/asphalt ratios.) Further, the diatomite-modified membrane mix is not as viscous as the asbestos membrane mix and, hence may be placed more easily and quickly using conventional paving equipment. Last, since diatomite disperses so well in asphalt mixes, it makes possible the addition of diatomite to cold mix pavements such as slurry seals. The improvements in cohesive/adhesive strength, tensile strength and mastic abrasion resistance rendered such a cold mix by diatomite, now make diatomite-modified slurry seals more attractive for a variety of uses including as a crack-resistant interlayer in the form of bridge deck and resurfacing underlayment membranes, as well as for economical resurfacing. When used in such cold mixes where elevated temperatures will not be used and, accordingly, the outgassing of water vapor which occurs in the hot mix will not occur in the same manner, pre-wetting of the diatomite is recommended in order to prevent premature absorption of the emulsion by the diatomite.
Additional features, characteristics and advantages of the present invention will become apparent after a reading of the following specification.